Film resistor and method of making same



Jan. 23, 1962 E. R. OLSON ETAL 3,018,198

FILM RESISTOR AND METHOD OF MAKING SAME Filed Aug. 15, 1959 INVENTORSEARL R. OLSON CHARLES M. CHAPMAN BY ,W

ATTORNEYS United States PatentO 3,018,198 FILM RESISTOR AND METHQD OFMAKING SAME Earl R. Olson, Rolling Hills, Calif., and Charles M Chapman,Columbus, Ohio, assignors, by mesne assignments, to Resistance ProductsCompany, Harrisburg, Pa., a corporation of Pennsylvania Filed Aug. 13,1959, Ser. No. 833,557 8 Claims. (Cl. 117-427) This invention relates toimproved film resistors and methods of making them. More particularly,it relates to metallic film resistors having a low temperaturecoefiicient of resistance, combined with a resistance which is high withrespect to that of similar resistors having metallic films of equalthickness.

Generally, materials for film-type resistors have relatively lowresistance per square. The resistance of a film can usually be increasedby decreasing the film thickness. However, as the films are madethinner, they become more susceptible to oxidation and the eifects ofmoisture and are usually very unstable. Also, such thin films often havehigh negative values of temperature coefiicient of resistance. Filmswhich are thicker and which may have a low temperature coefficient ofresistance and good stability, have low resistances per square. Theirproperties can be duplicated or bettered by resistive materials in wireform.

It may he mentioned that mere knowledge of properties of materials inthe bulk state has been found insufficient to enable accuratepredictability of the properties of those materials when in the form ofa thin film. Specifically, the properties mentioned below of an improvedfihn resistor may be neither measured nor predicted accurately fromproperties of the bulk material.

It is an object of the present invention to provide a film resistorwhich combines the properties of low temperature coefi icient ofresistance, high resistance, good control of the value of theresistance, and high stability.

It is another object of this invention to provide a superior method formaking the improved film resistor of the present invention.

Yet another object is to provide a superior resistive material in theform of a thin film.

Additional objects and features will become apparent as the descriptionproceeds.

The film resistors of the present invention are prepared by evaporatinga metallic bulk material in a high vacuum onto substrates of a ceramicmaterial, such as polished vycor glass or glazed ceramic. Similar vacuumevaporation processes for the production of film resistors are Wellknown. Of course, such variables as the temperature of evaporation,speed of rotation of the substrate, and degree of vacuum are determinedby the properties desired, the nature of the material to be evaporated,and by the desired thickness of the film.

According to the present invention, a thin film of a homogeneous metalmixture of at least two metalstis evaporated onto an insulating basematerial, the mixture including a substantial amount of at least onemetal which is an easily-oxidizable metal whose oxide is highly resistive, and the mixture including a substantial amount of at least oneconducting metal substantially more difiicult to oxidize than the easilyoxidizable metal. Then the deposited metal film is heat treated in anoxidizing atmosphere so as to selectively oxidize at least a substantialproportion of the easily oxidizable metal.

These steps greatly increase the resistance per square of the film byproviding many areas of high resistivity in the film with tortuousconductive paths in the film. After the heat treatment step, to insuregreater stability the film resistor may be protectively encapsulated topre- 3,018,198 Patented Jan. 23, 1962 vent further oxidation of thefilm. There is thus provided, in ametallic film resistor, a thinevaporated film comprising a metallic conducting matrix havingnonconducting portions substantially evenly distributed therein.

It has been found that the combination of desirable properties mentionedabove can best be attained by evaporating a homogeneous metal mixture ofplatinum and copper onto the film substrate. The highly preferred filmcan be produced from a binary alloy of platinum and copper, or from amixture of powdered platinum and powdered copper. The resultant film ithen heat treated at an elevated temperature in the presence of oxygenfor a brief period of time. During heat treatment, the resistance of thefilm has been found to increase to about 100 times the initial lowvalue. The composition of the film after heat treatment has been shownby electron diffraction measurements to be platinum-copper-copper oxide.

The above-mentioned properties are found in films produced from bulkmaterials ranging in composition by weight percent from 10 percentcopper to percent copper, the balance platinum. The composition of thefilm prior to heat treatment depends in part upon the method used forevaporating the bulk material. The film composition before heattreatment, with respect to the bulk material, will usually be slightlydeficient in platinum. Even heat-treated films prepared from aplatinumcopper alloy containing less than 10 percent by weight of copperhave comparatively low resistance per square. When the copper content ofthe bulk material is greater than 75 percent by weight, the filmproduced therefrom is not sufiiciently stable for usual resistorapplications. Preferred film compositions range from about 15 percent toabout percent copper, by weight, the balance platinum.

Films with excellent properties were produced, for example, from bulkalloy compositions of platinum-25 percent copper and platinum-5O percentcopper. Some spectrographic analyses of typical films produced fromthese alloys showed the metal content to be, respectively: platinum-30percent copper and platinum-65 percent copper. This latter compositionis highly preferred. Platinum-65 percent copper films may be used toobtain very high resistances, but are still stable and have a low temperature coefiicient of resistance.

In the drawings:

FIG. 1 is a perspective view of the apparatus used to evaporate films ofmetals onto cylindrical substrates, illustrating a preferred method.

A preferred method for evaporating the platinumcopper films ontosubstrates in the present invention, illustrated by FIG. 1, isevaporation in a vacuum system of a powdered alloy from a hot containeror boat, herein called a powder-hot-boat method. Such a method isdescribed in the Journal of Applied Physics, vol. 19, pages 739-741(August 1948). Several modifications to the method and of the devicetherein described were made. These modifications were necessary topermit the use of the high temperatures necessary to evaporate metalswith a high melting point.

In the powder-hot-boat apparatus of the present invention a tungstenboat or holder 10 is resistance heated to about 2100 to 230 0" C. Abovethe boat 10 are situated the substrates 11 on which the alloy is to beevaporated. When the boat 10 has reached temperature, a powder of thealloy to be evaporated is deposited into the hot boat 19 and the alloyis almost immediately evaporated onto the substrates 11.

In the present apparatus, the powder is deposited into the boat by ahopper 12. The hopper 12 is a capillary tube 13 flared at the entrance14 to permit ease of loading. The discharge of the powder is controlledby a small steel ball 15 pressed into the bottom of the capillary tube13 and afiixed to an extension 16 of the vibrating element of anordinaiy door-bell buzzer 17. To assure a good seal at the outlet, thesteel ball 15 is heated before being pressed into the end of the tube13. The hopper 12 and steel-ball valve 15 were made virtuallyselfaligning by allowing the hopper 12 freedom of vertical motion with aslight degree of lateral play. The rate of powder feed, determined inpart by the valve displacement, was most easily controlled by addingextra weights into the bowl of the hopper until the desired feed wasobtained. 7

The boat 10 was of tungsten strip 2 inches by /z-inch by -i'nch thickwith a large circular depression 18 in the center adapted to receive thepowder. The boats can be purchased or readily made from tungsten strip.At boat temperatures of about 2300 C., the current through the boat 10was about 300 amperes, necessitating the use of very heavy copperelectrodes 19 (connected to a power supply) to hold the tungsten strip.The power supply used provided this current from a -volt, 400-amperehookup.

A shutter 20 of aluminum was arranged so that it could be positionedbetween the boat and the substrates 11 to be coated while the boat 10was being brought to temperature. When the evaporation was about tobegin, the shutter was swung out of the way through a rotating seal 21in the base plate 22 of the evaporation chamber 23.

In making the evaporation, the bell-jar vacuum system was pumped down toabout 3Xl0- to 5 10- millimeters mercury. The substrates 11, supportedusually at 10 centimeters above the boat 10, were being heated duringthis time. With the shutter 20 in place, the boat 1 0 was slowly broughtto the desired temperature (determined by an optical pyromet'er), theshutter 20 was removed, and the powder was sprinkled onto the hot boat10 by activating the door-bell buzzer 17. A satisfactory rate ofdelivery of the powder was about 1 milligram per second. At theconclusion of the evaporation the shutter 20 was replaced. Air wasadmitted to the bell jar which the evaporation was performed after alapse of one-half hour, the period required for cooling of materialswhich became heated during evaporation.

A boat temperature of 2100" C. was suitable for platinum-base alloys.Very little loading of the boat with the alloy evaporated was observedin the powderliot-bo'at method of evaporation. v

Evaporation was made onto both cylindrical substrates, and flatsubstrates. In evaporating onto cylindrical substrates, as illustratedin FIG. 1, rotatable chucks 24 were supported over the powder-hot-boat10. The chucks 24 were rotated in unison by a variable speed motor. Therate of drive used was about 1500 r.p.m., although the optimum speed ofrotation was not deter mined. The substrates 11 were held in the chucks24 "and were heated prior to, and during, deposition of the alloy byradiation from a Nichi'orne heater. Evaporation was also made ontopolished flatVycor ceramic substrates. The substrate heater element forthe fiat substrates was a thin molybdenum sheet sandwiched between Vycorplates, one end of the unit being clamped to a support arm. The flatsubstrates were held against one surface of the heater unit by analuminum sleeve which gripped the extremities of the substrates.

The alloy to be evaporated was prepared by melting the constituents inan argon atmosphere by induction heating.

First a mixture of the constituent metal-s in powdered form wascompressed into a pellet. Then the pellet was inserted into a crucibleof spectrographically pure graphite for induction melting in argon. Thealloy was re tained in a molten state for about seconds to improve home'eneity.

All alloys were powdered by filing. A magnet was passed through "thefilings to remove any iron that may 4 have been removed from the file.The filings were then sieved to a particle size of IOU-mesh or less. Theiron content after filing was measured and was extremely small and isbelieved to have had little effect on the properties of the films.

Clean substrates are essential to the production and reproduction ofhigh-quality films. For the evaporations described herein, eachsubstrate was scrubbed with a softbristled brush wet with a detergentsolution. After a tapwater rinse, the substrates were allowed to soakfor onehalf hour in a hot saturated solution of CrO to which a smallamount of concentrated H 50 had been added. Most of the films were thenimmersed in concentrated HNO for a few minutes, then in a strongsolution of KOH for another few minutes. A very thorough rinse inrunning distilled water completed the cleaning operation until time touse the substrates for deposition of films. In some cases, the HNO andKOH steps were omitted with no observable difference in cleanliness. Themajority of the substrates were kept in distilled water until needed.Immediately prior to use, the surface to be filmed was flamed with thetip of a gas-oxygen flame. The substrates were not unduly heated by thisprocedure. The ionized gases at the tip of the flame were thought tohave a final cleaning effect on the surface of the substrates similar tothat of the ionized gases present in the glow discharge method of finalcleaning of films in an air atmosphere at reduced pressure. Somesubstrates were kept under methyl alcohol rather than distilled waterafter cleaning. The substrates appeared equally clean, irrespective ofthe storage solution. The final immersion in alcohol appeared to shortenthe pump-down time of the vacuum system, alcohol having a higher vaporpressure than water.

The preferred heat treatment for theplatinum-copper films of the presentinvention consists of heating to a temperature of from about 150 C. forat least about one hour to about 35 0 C. for at least about one minute.The range of possible useful film thickness is from about 50 angstromsto about 1500 angstroms. The heat treatment, of course, is conducted inthe presence of oxygen.

As an indication of the high stability and low temperature coelficientof resistance (TC) of these films, the following table is compiled fromdata gathered on typical heat-treated films of both Pt-25% Cu and Pt-50%Cu bulk alloy composition. As indicated, some of the films are onsubstrates of polished Vycor glass, others on glazed ceramic bases. Allwere on a load-life test for 1000 hours or longer. The load-life testconsisted of maintaining the films in air at an ambient temperature ofC., some of the films being encapsulated in glass. A direct current waspassed through the film to give an I R loss of one watt per square inchof film surface.

Film Sample Initial Final Percent Average v,(Bu'lk Alloy Base R/sq.R/sq. Change Composition) (ohms/ (ohms/ in 1,000 p.p.m./

square) square) hrs. 1 C.

It50% Ou. Vycor Glass s. 1,110 1,125 +1.4 45 Pt-50% Cu Glazed Ceramic...1, 280 1, 285 +0. 4 79 P's-50% Cu Vycor Glass 1, 470 1,493 +1. 6 52Pt50% Cu 0. 5,900 6,100 +3.4 64 Pt50% Cu 0 5,075 5, 300 +4.4 44 Pt 50%011..-- Glazed Ceramic..- 9, 600 9, 700 +1.0 19 mi -50% Cl1 do 13, 30013, 280 0. 2 34 Pt 25% Ou Vycor Glass 3,300 3,350 +1. 5 -11 Pt 50% Cu.do ,550 5, 570 +0. 4 62 1 Encapsulated.

It is well known that the resistance of films on sub strates of theproper geometry can be increased by spiral ing. By this technique, theresistance of a film on the outer surface of a cylindrical substrateinch long by inch diameter, for example, can be increased as much asthree thousandfold. Thus, such a film resistor of 5000 ohms resistancecan be altered to one of as high as 15 megohms resistance.

The effect of temperature in producing a permanent resistance increasewas examined for several of the platinumcopper (for example, platinum-25percent copper) films. A recorder was used in conjunction with aconstant current source to produce a continuous record of the deviationof resistance from the initial value as a function of temperature andtime. In most cases where the film was placed in a preheated oven at 200C., 250 C., or 300 C., resistance increased to a stable value Withinfive minutes. The increase was nearly linear with time. The higher thetemperature, the greater was the resistance change. That is, theresistance would become stable at a higher value of resistance. Wherethe film was placed in a cold oven and brought slowly to an elevatedtemperature, the resistance increase could be stopped at a predeterminedvalue, as indicated by the recorder, by removal of the film from theoven. Encapsulation and hermetic sealing at a stage during theresistance increase efi'ectively stops further oxidation of the copper,and a stable resistance is maintained at the predetermined level. At notime in working with this material was the temperature coefficient ofresistance measured to be greater than 100 parts per million per degreecentigrade.

The resistors of the present invention consistently provide films havingresistances per square greater than 1000 ohms while the temperaturecoefficients of resistance are less than 100 parts per million perdegree centigrade. There is thus provided, in a film-type resistor, ametallic film comprising at least 20 percent platinum, the film beingfrom about 50 angstroms to about 15 angstroms in thickness, which filmhas a resistance of at least 1000 ohms per square and yet a temperaturecoeflicient of resistance less than 100 parts per million per degreecentigrade.

Film resistors of other materials than copper and platinum were made andtested. The filament material for the evaporations described below wasTophet C, which consists of 61 percent nickel, 15 percent chromium, andthe balance iron, as manufactured by the Wilbur B. Driver Company, ofNewark, New Jersey.

The substrate material for the following described examples consisted ofAmerican lava-type ceramic designated Al Si Mag 196. The cylinders wereone inch long with a 4 -inch outer diameter and were glazed with aspecial glaze which matches the linear expansion (8.6 10' per C.) of theceramic material. A /s-inch-w1'de elec trode was fired onto each end ofthe cylinder, using a gold resinate manufactured by the Hanovia ChemicalWorks, of Newark, New Jersey. Immediately after this operation thesubstrates were ready for use without any further cleaning. Thesubstrate surfaces appeared to be smooth and free from contamination. Nosubstrate was heated before or during the evaporation process by anoutside source at any of the evaporations described below.

For Tophet C, all the test examples showed an increase in resistancefrom the value measured immediately after evaporation of at least 100percent after oxidation at 250 C. for 30 minutes. This increase, thoughsmall compared with the increase obtained with the platinum-coppersystem, could not be observed on films obtained under the samecircumstances with Tophet A, an alloy with the same percentageproportions of nickel and chromium as Tophet C, but containing no iron.Films with Tophet A alloy change approximately 1 percent after beingsubjected to the identical condition described above for Tophet C. Itthus appears that elective oxidation of the iron in Tophet C is at leastpartially responsible for the resistance increase.

It was suspected that a very irregular agglomeration of the chromium andnickel particles took place within these films, since the resistorsshowed a very high negative temperature coefficient of resistance.

Thus, although selective oxidation phenomena in film resistors mayappear with several combinations of materials, the preferred combinationof high resistance, stability, and low temperature coefficient ofresistance is found to an unusual degree in the preferred copperplatinumsystem.

It will be understood, of course, that the various forms of theinvention described herein are intended to be illustrative only and thatvarious changes may be made by those skilled in the art in practicingthe present invention.

What is claimed is:

1. A film-type resistor comp-rising a substrate having adherently fixedthereon a film consisting essentially of from about 15 percent to aboutpercent copper, by Weight, the balance platinum, said film being fromabout 50 angstroms to about 1500 angstroms in thickness.

2. A film-type resistor comprising a substrate having adherently fixedthereon a film consisting essentially of about 65 percent copper, byweight, the balance platinum, said film being from about 50 angstroms toabout 1500 angstroms in thickness.

3. A film-type resistor comprising a substrate having adherently fixedthereon a film consisting essentially of from about 15 percent to about80 percent copper, by weight, the balance platinum, that has beenheat-treated in an oxidizing atmosphere at a temperature of from about150 C. for at least about one hour to about 350 C. for at least aboutone minute.

4. A film-type resistor comprising a substrate having adherently fixedthereon a metallic film consisting essentially of from about 15 percentto about 80 percent copper, by weight, the balance platinum, said filmbeing from about 50 angstroms to about 1500 angstroms in thickness, andsaid film having a resistance of at least 1000 ohms per square.

5. In a film-type resistor, the film of claim 4, wherein said film has atemperature coeificient of resistance of less than parts per million perdegree centigrade.

6. A method of manufacturing film-type resistors which comprises:evaporating in vacuum at a temperature of at least 2000 C. a metalliccharge onto an insulating base to form a metalic film on said base, saidfilm having a thickness of from about 50 angstroms to about 1500angstroms, and said metallic charge consisting essentially of from about10 percent to about 75 percent copper, by weight, the balance platinum;and heat-treating said film in an oxidizing atmosphere at a temperatureof from about C. for at least about one hour to about 350 C. for atleast about one minute.

7. A method of manufacturing film-type resistors which comprises:applying, in a thickness of from about 50 angstroms to about 1500angstroms, a metallic film to an insulating base, said film consistingessentially of from about 15 percent to about 80 percent copper, byWeight, the balance platinum; and heat-treating said film in anoxidizing atmosphere at a temperature of from about 150 C. for at leastabout one hour to about 350 C. for at least about one minute.

8. The method of claim 7 wherein the film which is applied consistsessentially of about 65 percent copper, by weight, the balance platinum.

References Cited in the file of this patent UNITED STATES PATENTS1,717,712 Loewe June 18, 1929 2,387,970 Alexander Oct. 30, 19452,757,104 Howes July 31, 1956 2,803,729 Kohring Aug. 20, 1957

6. A METHOD OF MANUFACTURING FILM-TYPE RESISTORS WHICH COMPRISES:EVAPORATING IN VACUUM AT A TEMPERATURE OF AT LEAST 2000*C. A METALLICCHARGE ONTO AN INSULATING BASE TO FORM A METALIC FILM ON SAID BASE, SAIDFILM HAVING A THICKNESS OF FROM ABOUT 50 ANGSTROMS TO ABOUT 1500ANGSTROMS, AND SAID METALLIC CHARGE CONSISTING ESSENTIALLY OF FROM ABOUT10 PERCENT TO ABOUT 75 PERCENT COPPER, BY WEIGHT, THE BALANCE PLATINUM;AND HEAT-TREATING SAID FILM IN AN OXIDIZING ATMOSPHERE AT A TEMPERATUREOF FROM ABOUT 150*C. FOR AT LEAST ABOUT ONE HOUR TO ABOUT 350*C. FOR ATLEAST ABOUT ONE MINUTE.